There are various techniques of packaging a semiconductor die (microelectronic circuit), such as in a ceramic package, on a printed wiring board, in a plastic molded body, or on a tape. The latter two (plastic molded, tape) are representative of packaging techniques that employ lead frames. A lead frame is a generally planar layer of conductive material, patterned to have a plurality of conductor lines (lead fingers) radiating from a central area where the semiconductor die is located. For ceramic and printed wiring board packages, there is generally an analogous pattern of conductors on a insulating substrate (e.g., ceramic, FR4).
Irrespective of the type of package, the die must be connected to the inner ends of the lead fingers. One well-known way to effect this connection is by using bond wires attached at one end to bond pads on the die, and attached at their other end to the inner ends of the lead fingers. Bond wires are on the order of one thousandth of an inch, or less, in diameter. Bond pads are on the order of a few thousandths of an inch, spaced from one another on the order of one thousandth of an inch. Evidently, when many bond wires are connected to a die, this is a very crowded situation, and there is the possibility of these bond wires shorting against one another, breaking, and other related problems. This problem is exacerbated in plastic molded packaging, wherein a die is mounted to a lead frame, wire bonded thereto, inserted into a mold, and covered with plastic. The influx of plastic into the mold can cause movement of the closely-spaced bond wires, resulting in a defective packaged component. Coming this late in the process (during packaging), defects due to bond failure are generally irreversible and expensive.
The trend in microelectronics is to use smaller, denser semiconductor chips (dies) which have more functionality and which require more input/output (I/O) connections. Generally, each I/O connection requires a bond wire. It is therefore becoming increasingly difficult to package such chips (dies), especially in plastic packages, due to lead frame and bonding limitations.
FIGS. 1 and 2 illustrate a conventional technique of bonding a die 10 to a lead frame 13, such as for plastic packaging. Only a relevant, inner portion of the lead frame is shown in these illustrations. The lead frame 13 includes a plurality of conductive lines (lead fingers) 14 arranged in a generally radial pattern from a central area. The lead frame 13 further includes a die paddle 12, within the central area, and upon which rests the die 10. The die paddle 12 is preferably somewhat larger (in area) than the die. Tiebars 15 extend radially outward from the four corners of the die paddle 12 to an outer structure of the lead frame (not shown), to support the die paddle. As best viewed in the cross-section of FIG. 2 (tiebars omitted from this view), the die paddle 12 is depressed below the plane of the lead fingers 14. This is a fairly common practice, for plastic packaging (transfer molding). In FIG. 1, there are illustrated two of many bond wires 16, extending at one end 16a from a bond pad on a peripheral portion of the top face 11 of the die 10 to another end 16b connected to inner ends 14a of the lead fingers 14. As mentioned above, there may be hundreds of such bond wires, closely spaced, and subject to deformation during the plastic molding process.
If one is working at the lowest possible limit of lead resolution and spacing, the only way to accommodate more leads 14 is to move them further away from the chip--in other words to increase the gap between the die and the inner ends of the lead fingers. (As illustrated in FIG. 1, the lead fingers radiate, or spread out, from the central area containing the die. By terminating the inner ends of the lead fingers at a greater distance from the die, they are inherently further apart from one another.)
By moving the lead fingers further away from the die, it is evident that the bond wires need to be longer. Returning to FIG. 2, it is evident that the bond wires span a "gap" between the die 10 and the inner ends of the lead fingers 14. The bigger the gap, the longer the bond wires. Longer bond wires simply exacerbate the aforementioned problems of shorting, breaking, etc. Due to mechanical and molding considerations, the length of the bond wires 16 is the limiting factor in how far back one can position the leads. The maximum permissible single span of bond wire is on the order of 3-5 millimeters (mm), depending upon the application. Viewed another way, mechanical considerations vis-a-vis the (length of the) bond wires tend to limit the number of lead fingers, hence the I/O count, of a semiconductor device. This certainly seems antithetical in this day and age of miniaturization.